GLUCOSE STRUCTURE Flashcards
What is the molecular formula of glucose?
C₆H₁₂O₆
What happens when glucose is heated with HI?
It forms n-hexane, suggesting all six carbon atoms are in a straight chain.
What reactions confirm the presence of a carbonyl group (>C=O) in glucose?
Formation of oxime with hydroxylamine and cyanohydrin with HCN.
What indicates that the carbonyl group in glucose is an aldehyde?
Oxidation with bromine water forms gluconic acid (a six-carbon carboxylic acid).
What does acetylation of glucose with acetic anhydride confirm?
The presence of five –OH groups in glucose, forming glucose pentaacetate.
What happens when glucose or gluconic acid is oxidized with nitric acid?
Both yield a dicarboxylic acid, saccharic acid, indicating the presence of a primary alcoholic (–OH) group in glucose.
What does the ‘D’ in D(+)-glucose represent?
It represents the configuration of glucose relative to D-glyceraldehyde.
What does the ‘(+)’ in D(+)-glucose indicate?
It indicates that glucose is dextrorotatory (rotates plane-polarized light to the right).
Do ‘D’ and ‘L’ notations indicate optical activity?
No, ‘D’ and ‘L’ indicate relative configuration, not optical activity, and are unrelated to ‘d’ and ‘l’ (which denote actual optical rotation).
What do the ‘D’ and ‘L’ notations indicate in carbohydrates?
They indicate the relative configuration of a stereoisomer in relation to D- or L-glyceraldehyde.
What is the reference compound used to determine the D- or L-configuration of carbohydrates?
Glyceraldehyde, which has one asymmetric carbon and exists in two enantiomeric forms.
How do D- and L-glyceraldehyde differ?
They are mirror-image enantiomers; D-glyceraldehyde has the –OH group on the right, while L-glyceraldehyde has it on the left.
What determines whether a compound has a D- or L-configuration?
If it can be chemically correlated to D-(+)-glyceraldehyde, it has a D-configuration; if correlated to L-(–)-glyceraldehyde, it has an L-configuration.
How is the D-configuration identified in glyceraldehyde?
The –OH group is on the right when written using standard conventions.
How is the L-configuration identified in glyceraldehyde?
The –OH group is on the left when written using standard conventions.
Which carbon atom is used to determine the D- or L-configuration in glucose?
The lowest asymmetric carbon atom (farthest from the most oxidized carbon, –CHO).
Why is D-(+)-glucose assigned the D-configuration?
Because the –OH group on its lowest asymmetric carbon is on the right, similar to D-(+)-glyceraldehyde.
How should the structure of glucose be written for configuration comparison?
With the most oxidized carbon (–CHO) at the top.
Why doesn’t glucose give Schiff’s test?
Because glucose exists predominantly in its cyclic hemiacetal form, reducing the availability of the free aldehyde group.
Why doesn’t glucose form a hydrogensulphite addition product with NaHSO₃?
The aldehyde group in glucose is not freely available due to its predominant cyclic structure.
What does the pentaacetate of glucose not reacting with hydroxylamine indicate?
It indicates the absence of a free –CHO (aldehyde) group, confirming glucose exists in a protected cyclic form.
What are the two crystalline forms of glucose?
α-glucose and β-glucose.
How is α-glucose obtained?
By crystallization from a concentrated glucose solution at 303 K (melting point: 419 K).
How is β-glucose obtained?
By crystallization from a hot, saturated aqueous solution at 371 K (melting point: 423 K).
